Contrary to popular wisdom, water doesn’t conduct electricity very well. The lower the dissolved solid content (TDS), the greater its resistance to the flow of electricity. (Deionized water in its purest state has a resistance of 18.3 million ohms per cm.) In the water business we commonly use electrical resistance to describe the purity of deionized water. But whether the measure of purity indicated by a meter is in ppm, microsiemens, or ohms, it still derives from passing a small current through the water and measuring its resistance.

SWT's DI Lights measure the electrical resistance in the deionized water being tested. While that resistance is above a preset level, the green LED stays on. When the resistance of the water drops below that level, a red LED is turned on instead. These simple lights come in a wide range of preset values and are available with 1/2 inch MNPT or 3/4 inch MNPT connections. They are an effective and inexpensive way to monitor all but the highest quality deionized water. SWT's DI Lights are both accurate and intuitive in operation, as opposed to the confusing old on/off neon light where "on" indicated good, while "off" meant bad.